add domin for avatars.githubusercontent.com (#9425)

Author: lyndaidaii

src/NuGetGallery/App_Data/Files/Content/Trusted-Image-Domains.json

@@ -34,6 +34,7 @@
         "sonarcloud.io",
         "travis-ci.com",
         "travis-ci.org",
+        "avatars.githubusercontent.com",
         "raw.github.com",
         "raw.githubusercontent.com",
         "user-images.githubusercontent.com",